In recent years, development of techniques for compressing and encoding video signals, and larger capacities of storage media, such as magnetic disks or optical disks, have made it practicable to compress and encode video signals or sound signals and record the compressed and encoded signals in a storage medium.
FIG. 8 is a block diagram illustrating a prior art apparatus for decoding video signals. In the figure, reference numeral 1 designates an optical disk for recording bit streams that are obtained by compressing and encoding video signals and sound signals. When the bit streams are recorded in the optical disk 1, in order to prevent errors, interleaving is performed for the bit streams and error-correcting codes are added thereto. Reference numeral 2 designates an optical disk reading circuit for performing de-interleaving for a signal that is read from the optical disk 1 and correcting an error of the read signal, and outputting a bit stream 21. Reference numeral 8 designates a decoding circuit of video signals for decoding the bit stream 21 output from the optical disk reading circuit 2 to output a reproduced signal 81. Reference numerals 91 and 92 designate frame memories for storing the reproduced signal 81 that is produced in the decoding circuit 8. The reproduced signal 81 stored in the frame memories 91 and 92 is used as a reference image in the decoding circuit 8. Reference numeral 10 designates a frame memory for inputting and temporarily storing the reproduced signal 81 output from the decoding circuit 8, and reading out the reproduced signal 81 temporarily stored, according to a control signal from a reproduced signal output control circuit, thereby obtaining an output signal of this decoding apparatus. Reference numeral 12 designates a reproduced signal output control circuit for outputting a control signal required for readout of the reproduced signal 81 that is stored in the frame memory 10, to the frame memory 10.
The optical disk 1 in the apparatus for decoding video signals constituted as described above records video signals which are compressed and encoded according to the MPEG video standard of the ISO (hereinafter referred to as MPEG system). This MPEG system is a system of compressing and encoding video signals with high efficiency, based on interframe predictive coding. In this system, by changing prediction methods of respective plural frames, it is possible to realize high compression efficiency and reproduction in the middle of a sequence.
FIG. 9 is a diagram illustrating a plurality of consecutive image frames in time series. A prediction method in the MPEG system will be described. In the figure, numerals in the frames represent consecutive numbers of the frames, respectively, and it shows that time passes as the number increases. For example, the frame No.11 is a frame after the frame No.10.
In addition, alphabetical characters I, P, and B under the frames represent prediction methods of the frames, respectively. The frame with I, which is called I picture, is an intra coded picture that is encoded with only information in the frame. The frame with P, which is called P picture, is a predictive coded picture that is encoded by using the I picture or the P picture of three frames before as a reference image. For example, the P picture of the frame No.13 is predicted and encoded from one way using the P picture of the frame No.10 as a reference image. The frame with B, which is called B picture that is the characteristic of the MPEG system, is a bidirectlonally predictive coded picture that is encoded by using the frames before and after the B picture as a reference image. For example, the B picture of the frame No.12 is predicted and encoded from both way using the P pictures of the frames No.10 and No.13 as a reference image.
In the MPEG system, a group of a plurality of frames including one frame as I picture is called GOP (group of pictures). For example, in FIG. 9, a GOP unit comprises fifteen frames from the B picture of the frame No.2 to the P picture of the frame No.16, and corresponds to GOP(n), i.e., a range shown by a line with arrows at both ends. In addition, consecutive numbers n-1, n, n+1 of GOP are given for explanation. In the MPEG system, because the frames as P picture and B picture are predicted and encoded, based on data of the frames in one way and in two way, respectively, no perfect information is obtained from only data of the P picture or the B picture. Therefore, for example, random access to a GOP unit with data of a plurality of frames makes it possible to decode and reproduce a desired image from halfway through a serial video image. In this case, since the GOP unit includes one frame as I picture that is encoded with only information in the frame, decoding and reproduction are started from the I picture.
FIG. 10 shows a bit stream that is obtained by encoding the respective frames shown in FIG. 9. Since the order of the frames shown in FIG. 10 becomes one convenient for encoding processing and decoding processing, it is different from the order of the consecutive frames in time series shown in FIG. 9. For example, 41 represents the I picture of the frame No.4, 2B represents the B picture of the frame No.2, and 7P represents the P picture of the frame No.7. This notation will be used hereinafter when a specified frame and its picture type are described.
A description is given of operations of such an apparatus for decoding video signals as described above.
First, the operation in forward reproduction is described. The optical disk reading circuit 2 reads a bit stream from the optical disk 1 in order of the consecutive numbers of GOP. In the case of the bit stream shown in FIG. 10, that is in order of GOP(n-1), GOP(n), GOP(n+1), and GOP(n+2). The bit stream thus read is input to the decoding circuit 8. The decoding circuit 8 decodes the input bit stream sequentially to output the reproduced signal 81 according to the consecutive numbers of the frames. The frame memory 10 serves as a mere buffer, and temporarily stores the reproduced signal 81 and reads out the same, thereby producing an output 81 of this apparatus for decoding video signals.
Next, the operation in backward reproduction is described. The optical disk reading circuit 2 reads a bit stream from the optical disk 1 in reverse order of the consecutive numbers of GOP. In the case of the bit stream shown in FIG. 10, that is in order of GOP(n+2), GOP(n+1), GOP(n), and GOP(n-1). The bit stream thus read from the optical disk 1 is decoded in a forward direction by the decoding circuit 8 to obtain a reproduced image. The obtained reproduced image is stored in the frame memory 10. Then, the frame memory 10 reads out the reproduced image in a backward direction, thereby obtaining an output signal 81 of this apparatus, resulting in backward reproduction.
FIG. 11 is a diagram for explaining the operation of the frame memory 10 in backward reproduction. In the figure, GOP(n+1) shows the state in which a reproduced image obtained by decoding a bit stream of GOP(n+1) with the decoding circuit 8 is stored in the frame memory 10. Numerals 0 to 12 represent region numbers of the frame memory 10. This reproduced image, which is stored in the frame memory 10 with a storage region corresponding to thirteen frames, is read out, for each frame, from the region number 12 toward the region number 0, thereby obtaining an output signal 81 in which the reproduced image is in reverse of the forward order, like 31P, 30B, 29B, . . . . After outputting the reproduced image of the frames of GOP(n+1), a reproduced image obtained by decoding a bit stream of GOP(n) with the decoding circuit 8 is stored in the frame memory 10. In FIG. 11, GOP(n) shows this state. Then, the same operation as in the case of GOP(n+1) is performed, thereby producing an output signal 81 in which the reproduced image of GOP(n) is in reverse order. Hereafter, the operations described above are repeated to realize backward reproduction.
The prior art apparatus for decoding video signals constituted as described above requires a frame memory of a large capacity that can store a reproduced image of one GOP when backward reproduction of video signals is performed, leading to an increase in circuit scale and cost up.
Further, in the prior art decoding apparatus, when backward reproduction of video signals is performed, after the reproduced image of GOP(n+1) is output as an output signal from the frame memory, GOP(n) is decoded and the reproduced image of GOP(n) is stored in the frame memory 10 to produce an output signal. Therefore, there is a space of time between the output of the reproduced image of GOP(n+1) and the output of the reproduced image of GOP(n), whereby a space of reproduction time is produced between the frames which belong to the boundary between GOP(n+1) and GOP(n), for example, the frame 17B as the last output of GOP(n+1) and the frame 16P as the first output of GOP(n). As a result, it is difficult to obtain a smooth reproduced image.
Furthermore, when backward reproduction of video signals is performed in the prior art decoding apparatus, in the case of decoding B pictures which will make a reproduced image after I picture, for example, the frames 17B and 18B of GOP(n+1), the P picture, i.e., the frame 16P, of GOP(n) before GOP(n+1) including the reproduced image is used as a reference image. Therefore, another frame memory for storing a reproduced image of GOP(n) obtained by decoding GOP(n) is required, resulting in an increase in circuit scale of frame memories. In addition, when decoding of the frames 17B and 18B is omitted in order to avoid the increase in circuit scale, no perfectly consecutive frames are obtained, so that the quality of the image is adversely affected.